bpf, mips: No need to use min() to get MAX_TAIL_CALL_CNT

MAX_TAIL_CALL_CNT is 33, so min(MAX_TAIL_CALL_CNT, 0xffff) is always
MAX_TAIL_CALL_CNT, it is better to use MAX_TAIL_CALL_CNT directly.

At the same time, add BUILD_BUG_ON(MAX_TAIL_CALL_CNT > 0xffff) with a
comment on why the assertion is there.

Suggested-by: Daniel Borkmann <daniel@iogearbox.net>
Suggested-by: Johan Almbladh <johan.almbladh@anyfinetworks.com>
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/1661742309-2320-1-git-send-email-yangtiezhu@loongson.cn

diff --git a/arch/mips/net/bpf_jit_comp32.c b/arch/mips/net/bpf_jit_comp32.c
index 83c975d..ace5db3 100644 (file)
--- a/arch/mips/net/bpf_jit_comp32.c
+++ b/arch/mips/net/bpf_jit_comp32.c
@@ -1377,11 +1377,19 @@ void build_prologue(struct jit_context *ctx)
        int stack, saved, locals, reserved;
 
        /*
+        * In the unlikely event that the TCC limit is raised to more
+        * than 16 bits, it is clamped to the maximum value allowed for
+        * the generated code (0xffff). It is better fail to compile
+        * instead of degrading gracefully.
+        */
+       BUILD_BUG_ON(MAX_TAIL_CALL_CNT > 0xffff);
+
+       /*
         * The first two instructions initialize TCC in the reserved (for us)
         * 16-byte area in the parent's stack frame. On a tail call, the
         * calling function jumps into the prologue after these instructions.
         */
-       emit(ctx, ori, MIPS_R_T9, MIPS_R_ZERO, min(MAX_TAIL_CALL_CNT, 0xffff));
+       emit(ctx, ori, MIPS_R_T9, MIPS_R_ZERO, MAX_TAIL_CALL_CNT);
        emit(ctx, sw, MIPS_R_T9, 0, MIPS_R_SP);
 
        /*

diff --git a/arch/mips/net/bpf_jit_comp64.c b/arch/mips/net/bpf_jit_comp64.c
index 6475828..0e7c1bd 100644 (file)
--- a/arch/mips/net/bpf_jit_comp64.c
+++ b/arch/mips/net/bpf_jit_comp64.c
@@ -548,11 +548,19 @@ void build_prologue(struct jit_context *ctx)
        int stack, saved, locals, reserved;
 
        /*
+        * In the unlikely event that the TCC limit is raised to more
+        * than 16 bits, it is clamped to the maximum value allowed for
+        * the generated code (0xffff). It is better fail to compile
+        * instead of degrading gracefully.
+        */
+       BUILD_BUG_ON(MAX_TAIL_CALL_CNT > 0xffff);
+
+       /*
         * The first instruction initializes the tail call count register.
         * On a tail call, the calling function jumps into the prologue
         * after this instruction.
         */
-       emit(ctx, ori, tc, MIPS_R_ZERO, min(MAX_TAIL_CALL_CNT, 0xffff));
+       emit(ctx, ori, tc, MIPS_R_ZERO, MAX_TAIL_CALL_CNT);
 
        /* === Entry-point for tail calls === */